1. Field of the Invention
The present invention relates generally to wet cleaning of substrates during semiconductor wafer fabrication, and more particularly, to techniques for evaluating the effectiveness of techniques and apparatus used to dry substrates following a wet clean procedure.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform wet cleaning of substrates at various stages of the fabrication process. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed over and into silicon substrates. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. At each metallization level there is a need to planarize metal or associated dielectric material. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In some applications, metallization line patterns are formed in the dielectric material, and then metal CMP operations are performed to remove excess metallization.
Following each CMP operation, a wet clean of the substrate is performed. The wet clean is designed to wash away any by-products of the fabrication process, remove contaminants, and to achieve and maintain the necessary degree of cleanliness essential to proceed to a subsequent fabrication operation. As transistor device structures become smaller and more complex, the precision required to achieve and maintain structure definition demands exacting standards of cleanliness be maintained in all process operations. If a wet clean is incomplete or ineffective, or if a post-wet clean drying is incomplete or ineffective, then unacceptable residue or contaminants are introduced into the processing environment.
Rinsing and drying techniques, methods, and apparatus are plentiful and known in the art, and incorporate such operations as rinsing and scrubbing, immersion, and the application of thermal, mechanical, chemical, electrical, or sonic energy and the like to remove or displace water and dry the substrate. While some scrub and rinse operations may employ acids or bases for vigorous interaction with fabrication byproducts, deionized water (DIW) is commonly used to perform a final rinse before the desired drying technique is performed.
One common drying technique is known as spin, rinse and dry (SRD). SRD uses mechanical, centrifugal, energy to rid the substrate of water by spinning the substrate until dry. FIG. 1 shows a typical prior art SRD process and apparatus 10. An SRD apparatus 10 typically includes a substrate mounting plate 18 within a bowl 12 and mounted on a shaft 20 that is configured to rotate and thus spin the substrate 14. The substrate 14 is attached to the substrate mounting plate 18 with mounting pins 16 configured to maintain the substrate 14 in a horizontal orientation, firmly affixed to the substrate mounting plate 18 so that rapid rotation of the substrate mounting plate 18 spins the substrate 14 and forces the water from the substrate 18. DIW 26 is typically dispensed from a nozzle 24 which is positioned over the substrate 14 and connected to a DIW supply 22.
The SRD process essentially includes applying DIW or rinsing 28, and spinning the substrate dry 30. In some configurations, the substrate 14 is rinsed 28 while spinning to ensure thorough rinsing 28, and then spun to dry 30. The spinning of the substrate 14 uses centrifugal energy to force water from the substrate 14 surface, and can be enhanced with the introduction of an inert gas such as Nitrogen or an inert gas vapor to displace any water that is not completely removed by spinning. Additional variations include heating the DIW, heating the SRD environment, heating the inert gas, and the like.
Another common drying technique is known as a Marangoni technique. Marangoni drying (not shown) typically includes using a chemical drying fluid or solvent such as isopropyl alcohol (IPA) to introduce favorable surface tension gradients facilitating removal of water from the surface of a wafer. Variations of the Marangoni technique also include the introduction of an inert gas such as Nitrogen as a carrier gas for IPA vapor delivery.
Additionally, another known drying technique involves the replacement of DIW with another volatile compound.
Whichever method or combination of methods is employed to dry a substrate, effective drying is essential to continued fabrication. As is known, contaminates can damage or destroy features that are formed in single dies, groups of dies, or entire wafers.
Any water remaining of the surface of a substrate after the drying process evaporates. Water allowed to evaporate introduces contaminants as evidenced by the water marks or stains caused by residual solids from evaporated water. It is therefore desirable to evaluate drying techniques used, recognizing that the techniques are more or less effective depending on such factors as the type of substrate being processed, fabrication materials, processing environment, and the like. Common methods of evaluating the effectiveness of selected drying techniques include visual inspection, electrical analysis and mass analysis.
Visual inspection of substrates is generally effective for blanket film substrates as the surface of the substrate is smooth and easily inspected for remaining water marks. Patterned substrates, however, are difficult to inspect visually as water can be trapped in patterned features and not visible. Visual inspection is therefore ineffective for drying technique evaluation of patterned substrates.
Electrical analysis can be effective for specially prepared test structures after subjecting such structures to an electrical test such a TVS and the like. Such electrical analysis, however, is costly.
Mass analysis is a comparative evaluation of wet and dry substrates. Typically, mass analysis includes an initial drying operation followed by weighing the substrate and then, after some time, re-weighing the substrate to determine if a change in mass has or has not occurred. Although mass analysis is not subject to the same limitations presented by visual inspection and electrical analysis in the evaluation of patterned substrates, mass analysis is cumbersome, time consuming, and far less accurate than other methods.
What is needed is a method to evaluate advanced drying techniques used in the fabrication of semiconductor substrates. The method should include a way to accurately and precisely analyze a substrate that has been dried for any trace amount of residual contamination, and to use the results of the analysis to select, modify, or adjust the drying technique to ensure complete substrate drying in a contaminate-free environment.
Broadly speaking, the present invention fills these needs by providing a method for evaluating drying techniques. The method of evaluation includes applying a compound to a final rinse following a wet clean of a substrate, drying the wafer in accordance with the selected drying technique, and then analyzing any residual compound on the substrate after the drying method is completed. The present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, or a method. Exemplary embodiments of the present invention are described below.
In accordance with one aspect of the invention, a method for analyzing the effectiveness of a substrate drying operation is provided. The method includes applying a fluid solution that contains an analytically detectable compound to a surface of a substrate. The surface of the substrate has features defined therein. The method further includes spinning the substrate so that the fluid solution dries. When the fluid solution dries, it leaves a residue of the analytically detectable compound on and around the features in areas where the spinning of the substrate failed to completely remove the solution from the surface of the substrate during the spinning. The method then includes inspecting the substrate to identify any of the residue. Any residue that is identified defines those areas where the substrate drying operation failed to adequately remove the solution from the surface of the substrate.
In accordance with another aspect of the invention, a method for analyzing the effectiveness of a wafer drying operation is provided. The method includes applying a fluid solution that includes an analytically detectable compound to a surface of a wafer. The surface of the wafer has features defined therein, and the features define portions of semiconductor devices. The method next provides for spinning the wafer so that the fluid solution dries. The drying of the fluid solution is configured to leave a residue of the analytically detectable compound on and around the features in areas where the spinning of the wafer failed to completely remove the solution from the surface of the wafer during the spinning. The method further provides for optically inspecting the wafer. The optical inspection is designed to identify any of the residue which defines those areas where the wafer drying operation failed to adequately remove the solution from the surface of the wafer.
In accordance with yet another aspect, the present invention provides a method for evaluating substrate drying techniques. The method includes providing a substrate and applying an analytically detectable compound in solution to a first surface of the substrate. The method then provides for drying the substrate with the analytically detectable compound in solution on the first surface of the substrate, and then analyzing the first surface of the substrate to detect any residue of the analytically detectable compound remaining on the substrate.
In yet another embodiment, a method for rating the effectiveness of a wafer drying operation, is provided. The method includes applying a fluid solution including an analytically detectable compound to a surface of a wafer. The surface of the wafer has features defined therein, and the features define portions of semiconductor devices. The method next includes spinning the wafer so that the fluid solution dries. The drying of the fluid solution is configured to leave a residue of the analytically detectable compound on and around the features in areas where the spinning of the wafer failed to completely remove the solution from the surface of the wafer during the spinning. Next, the method provides for inspecting the wafer. The wafer inspection is designed to identify any of the residue. The identified residue defines those areas where the wafer drying operation failed to adequately remove the solution from the surface of the wafer. The method further includes modifying the wafer drying operation to improve drying, and repeating the method until an optimum drying performance is obtained for a specific wafer having predetermined geometric feature distributions.
In yet a further embodiment, a method for determining the effectiveness of a wafer drying operation is provided. The method provides for applying a fluid solution which includes an analytically detectable compound to a surface of a wafer that has features which define portions of semiconductor devices. The method then includes spinning the wafer so that the fluid solution dries. The drying of the fluid solution is configured to leave a residue of the analytically detectable compound on and around the features in areas where the spinning of the wafer failed to completely remove the solution from the surface of the wafer during the spinning. Next, the method provides for inspecting the wafer to identify any of the residue. Any identified residue defines those areas where the wafer drying operation failed to adequately remove the solution from the surface of the wafer. The wafer drying operation is then modified to improve drying and the method is repeated until an optimum drying performance is obtained for a specific wafer having predetermined geometric feature distributions. The method then implements the optimum drying performance in production wafer drying.
The advantages of the present invention are numerous. One notable benefit and advantage of the invention is the methods allow for non-biased, quantitative comparison of different drying techniques on patterned wafers. The most commonly utilized prior art provides no quantitative evaluation, and suffers significant shortcomings as previously detailed. The present invention can be implemented for a plurality of drying techniques, and provides usable, measurable data to evaluate the effectiveness of the selected technique for specific structures, geometries, complexities, and the like.
Another benefit is the cost effectiveness of the present invention. The methods do not require implementation with production, device wafers, but can be used in the RandD stage of production, and with test pattern wafers. This further allows effective evaluation of drying technologies at the stage of concept and feasibility studies, and thus reduces the associated cost of new cleaning tool development.
An additional benefit is that the present invention is an efficient and simple concept. Implementation is easily and efficiently incorporated into existing infrastructure, and vastly increases the efficiency of development and production.
Other advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.